Chemical vapor deposition precursors

ABSTRACT

Zirconium precursors for use in depositing thin films of or containing zirconium oxide using an MOCVD technique have the following general formula: Zr x (OR) y L, wherein R is an alkyl group; L is a β-diketonate group; x=1 or 2; y=2, 4 or 6; and z=1 or 2.

This application is a 35 U.S.C. §371 national phase entry ofPCT/GB98/01365 filed May 13, 1998.

DESCRIPTION

This invention concerns precursors for use in chemical vapour depositiontechniques the production of electro-ceramic devices therefrom, andtheir use in ferro-electric memories and I.R. detectors.

Metalorganic chemical vapour deposition (MOCVD) is a preferred methodfor depositing thin films, i.e. in the order of a few Am offerroelectric metal oxides, such as lead zirconate titanate,[Pb(Zr,Ti)O₃ or PZT and lanthanum-modified lead zirconate titanate.[(Pb,La)(Zr,Ti)O₃ or PLZT]. These electro-ceramic materials have a widerange of useful dielectric, ferroelectric, piezoelectric, pyroelectricand electrostrictive properties, giving rise to a variety of potentialapplications ranging from thermal imaging and security systems tointegrated optics and computer memories, e.g. DRAMS and non-volatileFERAMS.

The MOCVD technique involves transporting a metal as a volatilemetalorganic compound in the vapour phase followed by thermaldecomposition usually in the presence of oxygen on an appropriatesubstrate. The different types of substrate can be divided into threegroups, namely oxides, semiconductors and metals. Examples of suitableoxide substrates are SiO2, SrTiO₃, MgO and Al₂O₃. Semiconductorsubstrates include silicon (Si) and germanium (Ge) and metal substratesmay be, for example, molybdenum (Mo) or tungsten (W). MOCVD has a numberof advantages over other deposition techniques, such as sol-gel orphysical vapour deposition. MOCMD offers potential for large-areadeposition, excellent film uniformity and composition control, high filmdensities and deposition rates and excellent conformal step coverage atdimensions less than 2 μm. Furthermore, MOCVD processes are compatiblewith existing silicon chemical vapour deposition processes used in ULSIand VLSI applications.

Precursors for MOCVD of electro-ceramic the films are generally metalβ-diketonates, such as, for example, lead bis-tetramethylheptanedionate(Pb(thd)2, or metal alkoxides. WO 96/40690 discloses variousmetalorganic complexes of the formula MAyX, wherein M is a y-valentmetal, A is a monodentrate or multidentrate organic ligand coordinatedto M which allows complexing of MAy with X, y is a integer having avalue of 2,3 or 4 and X is a monodentrate, or multidentrate ligandcoordinated to M and containing one or more atoms independently selectedfrom C, N, H, S, O and F. A may be a β-diketonate and X may betetraglyme, tetrahyrofuran, bipyridine, crown ether or thioether.

It is important that the precursors are volatile enough to betransported efficiently at source temperatures which are below theprecursor decomposition temperature. In other words, there should be anadequate temperature window between vaporisation and decomposition. Theprecursors used need to be compatible and not pre-react. They shoulddecompose to form the desired metal oxide in the same temperatureregion. Ideally, precursors have low toxicity and are stable underambient conditions.

Available metal alkoxide and metal β-diketonate precursors generallyhave only very low vapour pressures, so that high source temperaturesare required for MOCVD. For example, Pb(thd)2 is typically transportedat above 130° C. and Zr(thd)4 at above 166° C. In conventional MOCVD inwhich a carrier gas is passed through a precursor held at a hightemperature for the duration of the deposition process, this can lead tothermal ageing, i.e. decomposition of the precursor prior to transportinto the reactor.

One way of avoiding this problem has been to use liquid injection MOCVD,in which a solution of the precursor(s) in an appropriate solvent, e.g.tetrahydrofuran, is evaporated and then transported to the substrate. Inthis way the precursor is only subjected to heating during evaporationrather than for the duration of the MOCVD process.

For ease of handling and volatility, toxicity and decompositioncharacteristics, the optimum precursor combination for MOCVD of PZT isPb(thd)₂, Zr(thd)₄ and either Ti(OPr^(i))₄ or Ti(OPr^(i))₂(thd)₂.However, there is a problem with using Zr(thd)₄, in that it is toostable, making it difficult to control the stoichiometry of PZT duringliquid delivery MOCVD. In particular, there is a large differencebetween the decomposition temperature of Zr(thd)₄ and the most usefullead precursor Pb(thd)₂. This results in a significant differencebetween the temperatures for diffusion (or mass-limited) oxide filmgrowth between the two precursors and the need to use high substratetemperatures to decompose the Zr(thd), source leads to a loss of leadfrom the PZT films by evaporation.

Zirconium alkoxides, such as Zr(OPr^(i))₄ and Zr(OBu^(t))₄ are predictedto be much less thermally stable than Zr(thd)₄ but are highly air andmoisture sensitive making them difficult to manufacture in pure form andtoo unstable for long term storage.

An object of this invention is to provide alternative Zr precursors foruse in MOCVD, especially for depositing PZT and PZLT.

Another object of the invention is to provide an improved method ofdepositing zirconium containing metal oxides in thin films.

According to a first aspect of this invention there is provided azirconium precursor suitable for use in MOCVD having the formula

Zr_(x)(OR)_(y)L_(z)

wherein R is an alkyl group

L is a β-diketonate group.

x=1 or 2

y=2, 4 or 6, and

z=or 2

According to a second aspect of the invention there is provided a methodof depositing thin films of or containing zirconium oxide usingmetalorganic precursors in an MOCVD technique, wherein the zirconiumprecursor has the formula

Zr_(x)(OR)_(y)L_(z)

wherein R is an alkyl group

L is a β-diketonate group.

x=1 or 2

y=2, 4 or 6, and

z=1 or 2

The preferred alkyl groups R are branched chain alkyl groups, preferablyhaving less than 10 carbon atoms, more preferably having 1 to 6 carbonatoms, especially iso-propyl and tertiary-butyl groups.

The preferred β-diketonate groups L include those of the general formula

wherein R¹ and R² are the same or different and are straight orbranched, optionally substituted, alkyl groups or, optionallysubstituted, phenyl groups. Examples of suitable substituents includechlorine, fluorine and methoxy.

Examples of suitable β-diketonate groups for use in precursors of theinvention include the following:

R¹ R² CH₃ CH₃ acetylacetonate (acac) CF₃ CH₃ trifluoroacetylacetonate(tfac) CF₃ CF₃ hexafluoroacetyl- (hfac) acetonate CH₃ C(CH)₃dimethylheptanedionate (dhd) C(CH)₃ C(CH)₃ tetramethylheptane- (thd)dionate CH₃ CF₂CF₂CF₃ heptafluoroheptane- (fhd) dionate C(CH)₃ CF₂CF₂CF₃heptafluorodimethyl- (fod) octanedionate CF₂CF₂CF₃ CF₂CF₂CF₃tetradecafluorononane- (tdfnd) dionate C(CH₃)₃ CF₃trifluorodimethylhexane- (tpm) dionate CF₃ CF₂CF₃octafluorohexanedionate (ofhd) C(CH₃)₃ CF₂CF₃ pentafluorodimethyl- (ppm)heptanedionate CF₃ CF₂CF₂CF₃ decafluoroheptanedionate (dfhd) C(CH₃)₃CH₂CH₂CH₂OCH₃ dimethylmethoxyoctane- (dmmod) dionate CCL₃ CH₃trichloropentanedionate (tclac) Ph Ph diphenylpropanedionate (dpp)

In one preferred embodiment of the invention the zirconium precursor hasthe following formula:

Zr(OR)₂L₂

wherein R and L are as defined above.

Typical examples of such zirconium precursors include Zr(OPr^(i))₂(thd)₂ and Zr(OBu^(t))₂(thd)₂

These compounds are believed to be particularly suitable for use in themethod according to the invention, especially in liquid injection MOCVD.

In another preferred embodiment of the invention, the zirconiumprecursor has the following formula:

Zr₂(OPr^(i))₆(thd)₂

Again this compound is believed to be particularly suitable for use inthe method of the invention, especially in liquid injection MOCVD.

Compounds of the invention may be produced by reaction of an appropriatezirconium alkoxide with an appropriate β-diketone.

The method of the invention is particularly useful for depositing on asubstrate thin films, i.e. in the order of up to 5 μm of lead zirconatetitanate (PZT) using a zirconium precursor according to the inventionwith a lead precursor, such as Pb(thd)₂ or lanthanum-modified leadzirconate titanate (PLZT). Typical substrates include SiO₂, Si, SrTiO₃,MgO, Al₂O₃, Ge, Mo and W.

According to a further aspect of the present invention there is provideda method of forming an electro-ceramic device comprising the steps ofdepositing a lower conducting electrode onto a substrate, depositing afilm layer of or containing zirconium oxide onto said electrode anddepositing an upper or further conducting electrode thereon, wherein thezirconium oxide layer is formed from the zirconium precursor having theformula:

Zrx(OR)yLz

wherein R is an alkyl group;

L is a β-diketonate group;

x=1 or 2;

y=2, 4 or 6; and

z=1 or 2.

The lower conducting electrode and upper conducting electrode ispreferably a metal, for example, platinum. The substrate is preferably asilicon wafer or circuit. An electro-ceramic device formed by thismethod is particularly suitable for use in ferro-electric memories andinfra-red detectors.

This invention will be further described with reference to theaccompanying drawings, in which;

FIG. 1 shows ¹H NMR spectrum for the product prepared in Example 1below;

FIG. 2 shows mass spectrometry results for the product prepared inExample 1 below;

FIG. 3 shows ¹H NMR spectrum for the product prepared in Example 2below;

FIG. 4 shows mass spectrometry results for the product prepared inExample 2 below;

FIG. 5 is a plot of growth rates against substrate temperature achievedby MOCVD using the products of Examples 1 and 2;

FIG. 6 shows a plot of growth rates against substrate temperatureachieved by MOCVD using a lead precursor;

FIG. 7 shows the likely chemical structure of Zr₂(OPr^(i))₆(thd)₂;

FIG. 8 is a plot of the growth rates against substrate temperatureachieved using the precursor Zr₂(OPr^(i))₆(thd)₂; and

FIG. 9 is a lateral cross-sectional view of an electro-ceramic deviceaccording to one embodiment of the present invention.

This invention will now be further described by means of the followingExamples.

EXAMPLE 1

Preparation of Zirconium di-isopropoxy bis-tetramethylheptanedionate

74 g of tetramethylheptanedionate were dissolved with stiring in 1 literof hexane in a 2 liter flask.

75 g of zirconium isopropoxide iso-propanol adduct were added to theflask and the mixture brought to reflux for 1 hour. The flask was cooledand the contents filtered through a pad and reduced in volume todampness using a Buchi roto-evaporator. The residue was redissolved in300 ml of hexane, clarified through a filter pad, stripped to halfvolume and 300 ml of dry isopropanol were added. The resultant solutionwas reduced in volume to 150 ml and set aside to crystallize thenfiltered off. The crystals were air dried or gently Buchi dried untilthe odour of isopropanol was removed.

The resultant product was relatively air stable, very soluble in hexaneand tetrahydrofuran, fairly soluble in ethanol and less in isopropanol.NMR and mass spectral analysis results for the product are shown inFIGS. 1 and 2 of the accompanying drawings respectively and themicroanalysis results are as follows:

Analysis % C % H Calculated 58.43 9.04 Found 56.86 8.30

These results indicate that the product had an approximate stoichiometryof Zr₂(OPr^(i))₂thd₂.

EXAMPLE 2

Preparation of zirconium di-tertiary-butoxybis-tetramethylueptanedionate

72 g of tetramethylheptanedione were dissolved with stirring in 1 literof hexane in a 2 liter flask. 74 g of zirconium tertiary butoxide wereadded to the flask (a slightly exothermic reaction) and the mixturebrought to reflux for 1 hour. The flask was cooled and its contentsfiltered through a pad before being reduced in volume to 200 ml using aBuchi roto-evaporator and set aside to crystallize. The resultingcrystals were filtered off and dried in air or gently Buchi dried tillthe odour of hexane was removed.

The product was air stable, very soluble in hexane and tetrahydrofuran,fairly soluble in ethanol and less in isopropanol.

NMR and mass spectral analysis results for the product are shown inFIGS. 3 and 4 of the accompanying drawings respectively. The results ofthe elemental microanalysis are given below:

Analysis % C % H Calculated 59.11 9.20 Found 58.66 8.70

These results indicate that the product had an approximate stoichiometryof Zr₂(OBu^(t))₂thd₂.

EXAMPLE 3 Deposition of ZrO₂ Thin Films.

Thin films of ZrO₂ have been deposited by liquid injection MOCVD withboth Zr(OPr^(i))₂(thd)₂ and Zr(OBu^(t))₂(thd)₂ in concentrations of0.09M in tetrahydrofuran. An evaporator temperature of 200° C. was usedwith argon flow of 4 liters/min and oxygen flow of 100-300 sccm. Growthrates achieved at different substrate temperatures are shown in FIG. 5of the accompanying drawings.

The suitability of either of these ZrO₂ precursors for use with atypical lead precursor, such as Pb(thd)₂ can be established from FIG. 6of the accompanying drawings which shows film lead oxide, including PbO₂growth rates from this lead precursor at different substratetemperatures. As can be seen from FIGS. 5 and 6 both the Zr and Pbprecursors provide optimum growth rates over a similar range ofsubstrate temperatures, i.e. from about 450-525° C.

It is believed that these Zr precursors are relatively stable to air andmoisture due to having six-fold co-ordination around the Zr centre, incontrast to the coordinately unsaturated Zr(OR)₄ compounds.

EXAMPLE 4

The product from Example 1 was recrystallized from n-hexane. Theresultant product had the stoichiometry of Zr₂(OPr^(i))₆(thd)₂ as shownin FIG. 7 of the accompanying drawings.

EXAMPLE 5 Synthesis of Zr,(OPr^(i))₆(thd)₂

Zirconium isopropoxide (2.93 g, 7.56 mmol) was dissolved in toluene(50cm³) and tetramethylheptanedionate (1.58 cm³, 7.56 mmol) was added.The solution was stirred at reflux for 1 hour after which time allvolatiles were removed in vacuo to yield a cubite solid. The white solidwas re-dissolved in toluene (20 m³) and left to stand at 0° C.overnight. Colourless crystals of Zr₂(OPr^(i))₆(thd)₂ were filtered off.

EXAMPLE 6 Synthesis of Zr₂(OPr^(i))₆(thd)₂

Zirconium isopropoxide (2.97 g, 7.25 mmol) was dissolved in n hexane (20ml) and tetramethylheptane—dionate (3.02 cm³, 14.5 mmol) was added. Thesolution was stirred at reflux for 1 hour after which time all volatileswere removed in vacuo to yield a white solid. This was re-dissolved inn-hexane (10 cm³) and left to stand overnight. The crystallisationprocess was repeated four times to yield colourless rhombohedralcyrstals which gave the single ray crystal X-ray structure ofZr₂(OPr^(i))₆(thd)₂. A proposed chemical structure forZr₂(OPr^(i))₆(thd)₂ is shown in FIG. 7 of the accompanying drawings.

Zr₂(OPr^(i))₆(thd)₂ is believed to be suitable for deposition of thinfilms of ZrO₂ by liquid injection MOCVD.

EXAMPLE 7

Growth Rates Achieved Using the Precursor of Example 6

Zr₂(OPr^(i))₆(thd)₂ has proved suitable for the deposition of thin filmsof ZrO₂ by liquid injection MOCVD. The films were grown using a 0.1molar solution of Zr₂(OPr^(i))₆(thd)₂ in tetrahydrofuran. An evaporatortemperature of 200° C. was used with a precursor injection rate of 3.5cm³hr⁻¹, an argon flow of 3000-5000 cm³ min⁻¹ and an oxygen flow of1000-2000cm³ min⁻¹. The growth rates achieved at different substratetemperatures are shown in FIG. 8 of the accompanying drawings, andindicate that ZrO₂ growth occurs over a significantly wider temperaturerange than is achievable with other precursors such as Zr alkoxides orZr(thd)₄.

It is believed that the novel Zr₂(OPr^(i))₆(thd)₂ source is moresuitable than existing Zr precursors for the MOCVD of Pb(Zr,Ti)O₃ andrelated ferro-electric materials at low substrate temperatures and ofyttria-stabilised zirconia at more elevated temperatures.

The zirconium precursors according to the present invention may be usedin the preparation of electro-ceramic device 2, as shown in FIG. 9 ofthe accompanying drawings. A lower conducting electrode 6, such asplatinum is deposited onto a substrate 4, such as silicon wafer orcircuit and a film layer 8 of a zirconium oxide is formed thereon usingthe zirconium precursor of the present invention. An upper conductingelectrode 10, which may also be platinum, is then deposited onto thezirconium oxide layer by appropriate deposition techniques. Theelectro-ceramic device may be used, for example, in ferro-electricmemories or infra-red detectors, such as those used in security lights.

What is claimed is:
 1. A zirconium precursor suitable for use in MOCVDhaving the formula: Zr_(x)(OR)_(y)L_(z) wherein R is an alkyl group, Lis a β-diketonate group, x=1 or 2, y=2, 4 or 6, and z=1 or
 2. 2. Azirconium precursor as claimed in claim 1, wherein R is a branched chainalkyl group.
 3. A zirconium precursor as claimed in claim 2, wherein Rhas less than 10 carbon atoms.
 4. A zirconium precursor as claimed inclaim 3, wherein R has 1 to 6 carbon atoms.
 5. A zirconium precursor asclaimed in claim 4, wherein R is selected from isopropyl and tertiarybutyl groups.
 6. A zirconium precursor as claimed in claim 1, whereinthe β-diketonate group L has the following formula:

wherein R¹ and R² are the same or different and are selected fromstraight or branched, optionally substituted, alkyl groups andoptionally substituted, phenyl groups.
 7. A zirconium precursor asclaimed in claim 6, wherein said optional substitaents are selected fromchlorine, fluorine and methoxy.
 8. A zirconium precursor as claimed inclaim 1 having the formula Zr(OR)₂L₂.
 9. The zirconium precursorZr(OP_(r) ^(i))₂(thd)₂, wherein thd is a tetramethylheptanedionategroup.
 10. The zirconium precursor Zr(OBu^(t))₂(thd)₂, wherein thd is atetramethylheptanedionate group.
 11. The zirconium precursorZr₂(OPr^(i)) ₆(thd)₂ wherein thd is a tetramethylheptanedionate.
 12. Amethod of depositing on a substrate a thin film comprising or containinga zirconium oxide precursor using a metal organic chemical vapordeposition technique, said process comprising the steps of: (a)introducing a zirconium precursor into a heated chamber through whichgas flows can be controlled to place the zirconium precursor in the gasphase; (b) transporting the zirconium precursor in the gas phase to adeposition chamber in which a substrate is present; and (c) heating thesubstrate to a temperature such that decomposition of the zirconiumprecursor decomposes on the heated substrate thereby depositing thedesired oxide thin film wherein the zirconium precursor has the formula:Zr_(x)(OR)_(y)L_(z)  in which R is an alkyl group, L is abeta-diketonate group, X is 1 or 2, y is 2, 4 or 6, and Z is 1 or
 2. 13.A method as claimed in claim 12, wherein one of the precursors is a leadprecursor.
 14. A method as claimed in claim 13, wherein the leadprecursor is Pb(thd)_(2.)
 15. A method as claimed in claim 12, whereinthe substrate is selected from SiO₂,Si, SrTiO₃, MgO, Al₂O₃, Ge, Mo andW.
 16. A method of forming an electro-ceramic device comprising thesteps of depositing a lower conducting electrode onto a substrate,depositing a film layer of or containing zirconium oxide onto saidelectrode and depositing an upper or further conducting electrodethereon wherein the zirconium oxide layer is formed from a zirconiumprecursor as claimed in claim
 1. 17. A method as claimed in claim 16,wherein the lower and/or upper conducting electrode is a metal.
 18. Amethod as claimed in claim 17, wherein the metal is platinum.
 19. Amethod as claimed in claim 16, wherein the substrate is selected from asilicon wafer or circuit.
 20. An electroceramic device formed by themethod of claim
 16. 21. An electro-ceramic device as claimed in claim 20for use in ferro-electric memories.
 22. An electro-ceramic device asclaimed in claim 20 for use in an infra-red detector.